short version on starting from scratch:
figure out your maximum compression clearance and maximum droop clearance.
in your case droop limited by driveshaft to frame, compression limited by bumpstops or tire clearance or driveshaft to frame or axle to fuel tank or whatever
it's [obviously] best to have bumpstops hit before shocks bottom, so extend if needed.
I'd then look at compressed shock lengths half an inch shorter than that, and see where that puts me for droop, and I'd probably center limit for the driveshaft to keep additional flex available for articulation if it's any sort of significant difference